Several decades of research have enabled the identification of curricula that prevent, or at least delay, substance use initiation. Research clearly shows that these curricula are most effective when they are implemented with fidelity;however, research also clearly shows that teachers often heavily adapt such curricula. As such, it is critical that we understand the nature of curriculum adaptations that teachers make, as well as the complex interplay between different types of adaptations and changes in key student outcomes such as attitudes and behaviors. The specific aims of this study are to: 1. assess the fidelity with which teachers implement two evidence-based substance abuse prevention curricula using four different assessment tools, with multiple methods of creating fidelity variables for each, and a detailed description of each tool, method of measurement, and its reliability;2. examine the relationship between fidelity of implementation and changes in student attitudes and behaviors using each of these tools that demonstrate adequate reliability, and 3. compare and contrast findings relative to the All Stars and Project ALERT (PA) curricula to identify the most useful assessment tools and associated measures for understanding changes in attitudes and behaviors on the part of students exposed to each The four different assessment tools specified in Aim 1 comprise detailed, step-by-step observer ratings;summary ratings by observers;expert judgments;and teacher self reports. Study data will be drawn from clinical trials of All Stars and PA in which all teachers implementing the curriculum videotaped their delivery, completed an implementation self-report survey, and surveyed students. Exploring our primary aims in reference to these two evidence-based curricula will allow us to determine the consistency of findings across the two, and thus make concrete recommendations as to how fidelity should be measured. Because teachers do make substantial modifications to curricula, a greater understanding of what elements of fidelity are most predictive of changes in student attitudes and behaviors will help researchers to develop more targeted and effective training tools, and thus ultimately enhance the effects or prevention curricula PUBLIC HEALTH RELEVACNE: Substance use prevention curricula are most effective when they are implemented as intended by the developer. Unfortunately, curriculum adaptation is the norm. Rather than trying to prevent this entirely, an effort that is likely futile, this study will help to identify what type of adaptations are helpful, harmful, or neutral. Such information can then be used as fodder for training programs to reduce harmful adaptations, ultimately leading to a more effective curriculum implementation and reduced adolescent substance use rates.